This invention relates to pulse radars and more particularly to pulse doppler and moving target indicating radars.
Doppler and moving target indicating (MTI) radars are used to detect targets in locations where return signals have a large clutter or noise content which interferes with determining the presence of a target using conventional radar signal processing techniques. Doppler radars can be capable of detecting moving targets under conditions in which conventional MTI processing is ineffective. Conventional MTI radar subtracts the received return signal from one pulse from the return signal for the next pulse as a means of removing the return signals from stationary objects (clutter) to leave only the returns from moving targets. Doppler radar relies on the frequency shift in the return signal which is produced by the velocity of a target which reflects the transmitted radar signal. In general, the objects which reflect the clutter components of the return signals are stationary or moving very slowly. As a consequence, clutter return signals are essentially at the same frequency as the transmitted radar signal. Where the target being searched for or tracked is a relatively high speed target, a sufficient doppler frequency shift occurs in the signals reflected from that target to enable them to be separated from the clutter signals by narrow band filtering.
As a consequence of these characteristics doppler radar has become of increasing interest with the development of high speed aircraft which are capable of flying close to the ground. Such an aircraft may escape detection by conventional radar and MTI radar because clutter signals reflected from the ground, vegetation or the surface of the sea may be strong enough to interfere with the detection of the signals returned from the aircraft. These problems are greatly reduced through the use of doppler radar. However, as with all radars, doppler radars are subject to electronic counter measures (ECM) techniques such as jamming and signal repetition and must contend with noise. One technique which can be used to reduce the problem of close-in clutter is sensitivity time control (STC) in the receiver. With STC, the sensitivity of the receiver is kept low after the transmission of a pulse until returns from close-in clutter have been received and then the sensitivity is increased to receive returns from more distant objects which are of interest. This prevents overloading both the receiver and signal processing system with close-in clutter returns which are of no interest.
One limitation on radar immunity to ECM which is encountered with both MTI and doppler radars is the N-time-around problem. The N-time-around problem is created when additional radar pulses are transmitted before returns from distant targets are received from a first pulse. Under these conditions returns from distant targets illuminated by earlier transmitted pulses are received intermixed with returns from nearer targets illuminated by subsequent pulses. This produces several problems. One is ambiguity as to the range of a target since any given return may be from as many different ranges as there are transmitted pulses from which returns can currently be received. Another is that STC can not be used to reduce close-in clutter returns without also reducing sensitivity to target returns from earlier transmitted pulses. An ECM problem arises because of the possibility of a jammer receiving a transmitted pulse and repeating it in a manner to create spurious targets and/or mask targets by narrow band jamming at the transmitted frequency.
A means is needed of minimizing the vulnerability of a doppler or MTI radar to ECM. One way of accomplishing this is transmitting a signal which forces a jammer to jam a wide frequency band rather than just the very narrow frequency band in which the radar is actually operating. This requires transmission of a signal which prevents the jammer from merely retransmitting the signal as a means of jamming the radar receiver. Forcing a jammer to jam a wide frequency range can result in a 18 dB reduction in jammer power at the frequency the radar is actually operating at when the jammer jams a frequency band which is 69 MHz wide and the radar is only utilizing a 1 MHz bandwidth. Another technique is to use frequency agility to rapidly change the transmitted frequency as a means of reducing the effectiveness of signal repetition ECM techniques.
It is desirable to provide a radar which provides the benefits of MTI or doppler processing while ensuring that narrow band jamming and signal repetition will be ineffective ECM techniques while enabling the use of STC clutter reduction techniques and eliminating the N-time-around problem.